Heating and/or cooling circulating baths are used in laboratory settings for providing a controlled temperature fluid, such as water. The end user may utilize the circulating bath by placing their application in the bath reservoir or by circulating the reservoir fluid between the bath reservoir and an external application. Conventional circulating bath applications may include placing beakers or other containers in the bath reservoir or controlling the temperature of an external jacketed vessel. The circulating bath moves the fluid past heating or cooling elements located in the bath reservoir so as to achieve a uniform desired fluid temperature.
Circulating baths typically operate over temperatures ranging from −50° C. to +300° C., depending on the configuration of the bath and its intended application. To prevent overheating and/or other dangerous conditions, circulating baths typically include a low fluid level detection device to shut down the system heater if the reservoir runs dry. The circulating bath will also typically include a high temperature limit switch to prevent the fluid or surfaces within the circulating bath from exceeding a safe temperature. The low fluid level detection device and high temperature limit switch thus provide two levels of safety for preventing the circulating bath from overheating. The high temperature limit switch typically includes a temperature sensing bulb that is attached to the top of a heater coil to monitor both heater and fluid temperatures. As the temperature of the bulb rises, fluid located within the bulb expands. This expanding fluid is typically coupled through a capillary tube to a diaphragm located within the switch housing. When the temperature of the expanding fluid exceeds a set-point temperature, the pressure exerted by the expanding fluid causes an electrical contact within the limit switch to open. Thus, if the temperature of the fluid in the bulb exceeds the set-point temperature, the resulting pressure trips the limit switch. Because the reservoir heater is coupled to power through the temperature limit switch, the temperature limit switch turns off the heating element within the circulating bath when the limit switch opens.
In order to be certified by Underwriters Laboratory, a circulating bath must meet the requirements of UL 61010-1 entitled “Electrical Equipment For Measurement, Control, and Laboratory Use” and in particular, Section 9.4 thereof entitled “Requirements for Equipment Containing or Using Flammable Liquids.” This standard requires the maximum temperatures measured during testing to be at least 25° C. below the fire point of the recommended fluids for the circulating bath. The test may be conducted with one safety device at a time disabled, or placed in a fault condition, to determine if a single failure could result in a dangerous over-temperature condition. To create the highest possible surface temperatures that would be encountered during operation of the circulating bath, a test is done with a dry reservoir. The dry reservoir test simulates a situation that might arise if an operator forgot to add fluid to the reservoir or the fluid has drained from the reservoir. To simulate a failure of the reservoir level switch during this test, the reservoir level switch is faulted in the up, or full, position to provide a false indication that there is fluid in the reservoir.
During a reservoir level switch fault test, thermocouples for monitoring system temperatures are typically mounted to the heater near the high temperature limit bulb, and to other locations that may be expected to experience excessive temperatures based on a visual inspection of the circulating bath. The circulating bath is turned on with the reservoir empty. All thermocouples are then monitored for maximum temperatures achieved from the time the circulating bath is activated until after the high temperature limit switch has opened and the circulating bath has shut off. If the high temperature limit switch is working properly, the measured temperatures will not exceed the maximum allowable temperature for the particular fluids.
Because the high temperature limit bulb has thermal mass, the temperature of the sensing bulb may lag the temperature of the heating element, fluid, and/or other surfaces in the circulating bath. This lag may result in a temperature overshoot by the heating element due to the difference between the temperature of the heating element and the temperature of the sensing bulb. Therefore, to ensure that surface and fluid temperatures do not exceed the maximum allowable temperatures, it may be necessary to configure the high temperature limit switch to activate at a set-point temperature that is well below the maximum allowable temperature. Because of the rapid heating of high wattage density heating elements, thermal lag may result in significantly reduced operating temperatures. For example, when using silicone oil as a bath fluid, the set-point temperature may need to be 60° C. to 80° C. below the fluid fire point in order to meet the requirements of UL 61010-1 for a compact circulating bath. This problem may be further exacerbated by the reduced surface dimensions of the high temperature limit bulbs used in these compact circulating baths.
Thus, there is a need for improved systems and methods for monitoring the conditions of circulating baths to prevent the circulating bath from overheating.